1. Technical Field of the Invention
The present invention relates to a mold for the injection molding of parts made of plasticizable material, in particular made of plasticizable liquid crystal polymers, wherein the mold comprises at least two gates, each of which has an assigned plasticizing and injection unit.
2. Discussion of the Related Art
As used herewith, plasticizable material encompasses the usual plasticizable thermoplastics; granulated mixtures that are completely reacted into polyurethane and comprise isocyanate and polyol; unburned ceramic compounds with added organic lubricants; elastomers; filled plastic material further comprising added glass fibers, carbon fibers, metal powders, etc.; and other known plasticizable materials.
To date, when injection molded parts are injection molded, plasticized material is generally injected into the cavity of a closed mold by means of a gate, and the shrinkage that can not generally be neglected when the injection molded part is cooling is compensated for by the further addition of plasticizable material. According to this general principle, injection molded parts are also manufactured of different plastic materials, i.e., multi-component injection. Since, apart from the gate, plasticized material is always injected into a self-contained cavity depending on the geometry, the dimensions, and the wall thickness of the injection molded part, significant injection pressures for manufacturing faultless injection molded parts and also correspondingly high mold clamping pressures are necessary in order to prevent the mold from opening.
To date attention has been paid substantially to controlling or regulating the sequence of the injection process to manufacture faultless injection molded parts that are true to size or dimensions. However, with these measures it is not possible and also not intended for the plasticized material to flow specifically into the cavity for the purpose of a predetermined orientation, e.g., of the neutral fibers to result in optimal mechanical strength or load-bearing capacity of the injection molded parts.
When injection molded parts such as push rods for motor vehicles, etc. are manufactured, it is known from European patent application 0205 710 to use molds with several gates and to provide a plasticizing and injection unit for each gate respectively in order to avoid a reliable filling of the mold during the injection operation and, moreover, to avoid freezing of the plasticized material in the gates.
Furthermore, it is known from European patent application 0166 038 to manufacture injection molded parts of plasticized liquid crystal polymers, whose molecular chains orient themselves in the flow direction of the plasticized material injected into the mold. To this end, molds with two gates are used that form a closed system with a plasticizing and injection unit so that when plasticized material is injected into the cavity, it flows in part into the second gate after the mold has been filled and the molecular chains are oriented in the cavity by means of the resulting laminar current.
From other fields of technology, e.g., in the case of forged pieces, it is known to target an optimal strength or load-carrying capacity of the tool through a suitable setting of the fiber flow.
A process to injection mold parts made of plasticizable material, in particular plasticizable liquid crystal polymers, is known from the West German Patent application P 38 10 954.9, which corresponds to U.S. Pat. No. 4,994,220 the specification of which is hereby specifically incorporated by reference. The plasticized material is injected into a closed mold with at least two gates, each of which has an assigned plasticizing and injection unit. Any shrinkage of the injection molded parts that are cooling down is compensated for through an additional optional supply of material. To fill the cavity of the plasticized material the plasticized material, flows through the one plasticizing and injection unit via the one gate into the cavity and continues to flow in part via the other gate in the direction of the other plasticizing and injection unit. To set a specific fiber orientation or stress curve, the two plasticizing and injection units feed alternatively plasticized material to the cavity while the cavity is being filled.
The plasticized material flows into the cavity through the one gate and, during the filling of cavity, alternatively flows through the other gate in the direction of the other plasticizing and injection unit. Thus, the hydraulic pressures acting on the plasticizing and injection screws are specified in such a manner that the plasticizing material is under a set pressure. While the one plasticizing and injection screw moves in the direction of its assigned gate, the other plasticizing and injection screw moves away from its assigned gate, whereby it can also plasticize granulated material. Upon filling the mold, further axial movement of both plasticizing and injection screws is stopped and a corresponding pressure, under which the injection molded part cools with or without compensation for shrinkaqe, is applied to one or both plasticizing and injection screws. At the same time, a build-up of the usual high mold internal pressure during filling of the cavity is avoided so that even the previously usual high mold clamping pressures are no longer necessary.
It is especially advantageous if, through the specific arrangement of the two gates with respect to the geometry and dimensions of the cavity, a specific flow pattern of the plasticized material in the cavity is achieved and the orientation of the fibers, in particular the orientation of the neutral fibers, can be specified. Another significant advantage lies in the fact that long molecules or molecular chains and glass fibers, carbon fibers etc orient themselves in the direction of flow and thus significantly increase the part strength.
This alternate feeding can take place in such a manner that the two plasticizing and injection units feed the entire requisite mass to manufacture an injection molded part in partial quantities to the cavity wherein the flow direction reverses from time to time. Especially with long molecular chains, orientation in the flow direction occurs without more effort due to the multiple reversal of direction of the flow direction. If, upon filling the cavity by means of the two plasticizing and injection units, the plasticized material is moved back and forth as a function of the cooling rate of the injection molded parts, the injection molded part cools down from the mold inner wall in the direction of the interior of the injection molded part. In this case shear sections are formed in which the orientation of the molecules or molecular chains takes place so that in the ideal case an orientation of the molecules and the targeted flow of the fibers over the entire cross section is achieved.
The intervals for moving the plasticized material back and forth can generally be separated from one another through pauses so that the speed of the material flowing back and forth can be controlled over a wide range due to the mass moment of inertia to achieve a surprisingly optimal orientation of the molecules or the molecular chains.
In principle it is possible to arrange the two gates of a closed mold diametrically opposite one another so that the cavity has a function similar to a gate while the plasticized material is flowing. Such a design is especially advantageous for injection molded parts having cylindrical or rotational symmetry.
In general, the gates car be arranged in any arbitrary manner with respect to one another and, in particular, take into account the geometry of the cavity so that the correspondingly specified fiber orientation or stress curve in the injection molded part is obtained. Thus, for example, when L-shaped profiles are being extruded, it is advantageous to have the axes of the gate intersect at a 90.degree. angle so that when the L-shaped profiles are injection molded the fibers have an L-shaped orientation.
Plasticized material for changing the flow direction in the cavity can be fed in particular over at least one other pair of gates alternately to the first pair of gates so that through these measures the injection molded part is extruded in two layers. By means of the suitable arrangement and orientation of the second pair of gates it is possible to set the fiber orientation or stress curve of the second layer in such a manner that it is displaced by 90.degree. with respect to the direction of the fiber and/or the stress of the first layer.
In principle it is possible to extrude injection molded parts comprising several layers according to the principle of the multilayered plate by means of a suitable arrangement of additional pairs of gates, where the fiber orientation of the successive layers is staggered with respect to one another. In principle it is also possible to orient pairs of gates in successive planes and alternately in different directions and then to extrude the injection molded part in successive layers.
Furthermore, while the plasticized material is flowing in the direction of the one or the other plasticizing and injection unit, the cavity can be vented. The plasticized material flowing into the cavity and flowing or in the direction of the other plasticizing and injection unit moves the air located in the cavity in front of itself and delivers it on its flow-way at a suitable point via a valve to the environment.
In principle, in the simplest case at least two plasticizing and injection units and a mold with at least two gates are used, wherein to extrude the injection molded parts the cavity of the mold is connected so as to communicate with the plasticizing and injection units via the gates. By communicating connection is understood that the plasticized material of both plasticizing and injection screws is injected alternately into the cavity and also flows in a controlled manner alternately in the direction of one or the other plasticizing and injection unit. With these measures the plasticized material can be fed to the cavity with a set pressure differential exerted by the plasticizing and injection screws.